Diesel combustion exhaust is a regulated emission. Diesel particulate matter (PM), is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, PMs can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers. Various technologies have been developed for identifying and filtering out exhaust PMs before the exhaust is released to the atmosphere.
As an example, soot sensors, also known as PM sensors, may be used in vehicles having internal combustion engines. A PM sensor may be located upstream and/or downstream of a diesel particulate filter (DPF), and may be used to sense PM loading on the filter and diagnose operation of the DPF. Typically, the PM sensor may sense a particulate matter or soot load based on a correlation between a measured change in electrical conductivity (or resistivity) between a pair of electrodes placed on a planar substrate surface of the sensor with the amount of PM deposited between the measuring electrodes. Specifically, the measured conductivity provides a measure of soot accumulation. As such, the sensitivity of the PM sensors to measure PM in the exhaust may depend on the exhaust flow rate, with increased exhaust flow rate leading to increased PM sensor sensitivity and decreased exhaust flow rate resulting in decreased PM sensor sensitivity. With this increased dependence on exhaust flow rate, the PM sensor capturing the PMs exiting the DPF, may not truly reflect the DPF filtering capabilities. Furthermore, PM sensors may be prone to contamination from impingement of water droplets and/or larger particulates present in the exhaust gases, thus affecting the PM sensor sensitivity and leading to errors in the output of the PM sensor.
One example PM sensor design is shown by Nelson in U.S. Pat. No. 8,225,648B2. Therein, a PM sensor includes a flow redirector and a barrier positioned around a PM sensor element to filter out the larger particulates from impinging the PM sensor element. The barrier thus serves to block larger particulates in the exhaust flow from impinging on the PM sensor element, thereby reducing PM sensor sensitivity fluctuations due to large particulates depositing on the PM sensor element.
However, the inventors herein have recognized potential issues with such an approach. As one example, the PM sensor sensitivity may continue to depend on the incoming exhaust flow rate. In one example, the issues described above may be partly addressed by a method for adjusting an amount of opening of an inlet to a particulate matter sensor positioned in an exhaust flow in response to an exhaust flow rate of the exhaust flow upstream of the particulate matter sensor, the particulate matter sensor element oriented with its major surface parallel to a direction of exhaust flow. In this way, the sensitivity of the particulate matter sensor may become independent of the exhaust flow rate and the PM sensor output may begin to measure the DPF filtering capabilities more accurately and reliably.
As one example, when the exhaust flow rate falls below a threshold, the amount of opening of the inlet of the PM sensor may be increased to allow more exhaust gas into the PM sensor for subsequent deposition on a PM sensor element positioned inside the PM sensor. When the exhaust flow rate rises above the threshold, the amount of inlet opening may be decreased to reduce the exhaust gas entering the PM sensor. Herein, the increasing and the decreasing of the amount of inlet opening may be regulated by adjusting (e.g., rotating) a movable flow controller positioned at the inlet. In this way, the amount of exhaust gas and thereby the amount of particulates getting deposited on the PM sensor element positioned proximate to an outlet of the PM sensor may become independent of the incoming exhaust flow rate, thereby measuring PMs exiting the DPF more accurately and reliably. Further, larger particulates and/or water droplets may be trapped by the first flow redirector. The PM sensor element may be positioned parallel to the first flow redirector and a second flow redirector, with a narrow passage located between the PM sensor element and the second flow redirector. Therefore, the PM sensor element may be protected from impingement of water droplets and larger particulates while attracting smaller particulates to accumulate onto one of the major surfaces of the PM sensor element comprising electrodes. Overall, these characteristics of the sensor may cause an output of the PM sensor to be more accurate, thereby increasing the accuracy of estimating particulate loading on a particulate filter.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.